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Anal. Chem. 1982, 5 4 , 2154-2158
posited in a surface pore in the case of NPG. The authors further state that relative to the use of PG, the use of NPG contributes to an increase in the effective vaporization temperature of an element, also a fact that would fit with the data discussed here. The results obtained from the nickel determinations further confirm the results obtained with lead. The interferences due to the MgClz matrix are not as severe as those seen for P b presumably due to the fact that the appearance temperature for Ni is much higher than that for Pb. Again, the interferences on the PG surface are much more severe than those on the NPG surface. These results confirm the earlier statement that the use of a NPG surface may be very useful in minimizing some matrix effects.
LITERATURE CITED Frech, W.; Cedergren, A. Anal. Chlm. Acta 1976, 82, 93. Fuller, C. W. Anal. Chlm. Acta 1978, 81, 199. L'vov, B. V. Specfrochim. Acta, Part8 1978, 338, 153. Czobik, E. J.; Matousek, J. P. Anal. Chlm. Acta 1976, 5 0 , 2. Segar, D. A.; Gonzalez, J. G. Anal. Chim. Acta 1972, 58, 7 Churella, D. J.; Copeland, T. R. Anal. Chem. 1978, 50, 309. Krasowski, J. A.; Copeland, T. R. Anal. Chem. 1979, 51, 1843. Erspamer, J. P. Ph.D. Dlssertatlon, University of New Mexico, Albuquerque, NM, 1980. Erspamer, J. P.; Nlemczyk, T. M. Appl. Spectrosc. 1981, 35, 512.
(10) Sturgeon, R. E.; Chakrabarti, C. L.; Langford, C. H. Anal. Chem. 1976, 4 8 , 1972. (11) van den Broek, W. M. G. T.; de Galan, L. Anal. Chem. 1977, 49, 2176. (12) Paverl-Fontana, S. L.; Tessari, G.; Torsl, G. Anal. Chem. 1974, 46, 1032. (13) Torsl, G.; Tessari, G. Anal. Chem. 1975, 4 7 , 839. (14) Sturgeon, R. E.; Chakrabarti, C. L. Anal. Chem. 1977, 49, 1100. (15) Fuller, C. W. Analyst (London) 1974, 99. 739. (16) Maessen, F. J. M . J . ; Balke, J.; Massee, R. Specfrochlm. Acta, Part B 1978, 338, 311. (17) Sedykh, E. M.; Belyaev, Yu, I.; Dzhegov, P. I. Zh.Anal. Khlm. 1979, 34 1984 (18) SaimOn, S. G.; Davis, R. H., Jr.; Holcombe, J. A. Anal. Chem. 1961, 53, 324. (19) Slavln, W.; Carnrick, G. R.; Manning, D. C. Anal. Chem. 1962, 54, 621. (20) Lawson, S. R.; Woodriff, R. Specfrochlm. Acta, Part B 1980, 358, 753. (21) Slemer, D. D. Anal. Chem. 1982, 52, 1659. (22) Hocquellet, P.; Lebeyrle, N. Analysis 1975, 3 , 505. (23) Sturgeon, R. E.; Chakrabartl?C. L. Anal. Chem. 1977, 49, 90. (24) Slavin W.; Manning, D. C.; Carnrlck, G. R. Anal. Chem. 1961, 53, 1504. (25) Erspamer, J. P.; Nlemczyk, T. M. Anal. Chem. 1982, 5 4 , 538. (26) Czobik, E. J.; Matousek, J. P. Anal. Chem. 1978, 5 0 , 2. (27) Ottaway, J. M. Proc. Anal. Chem. SOC. 1976, 13, 185. (28) L'vov, 6 . V.; Bayunov, P. A.; Ryabchuk, G. N. Spectrochim. Acta, Part B 1981, 368, 397.
RECEIVED for review March 9,1982. Accepted August 2,1982.
Determination of Calcium, Magnesium, Strontium, and Silicon in Brines by Graphite Furnace Atomic Absorption Spectrometry Linda A. Powell" and Richard L. Tease E.
I. du font de Nemours l? Company, Polymer Products Department, Experimental Station, Wilmington, Delaware 19898
Graphite furnace atomlc absorption spectrometry has been lnvestlgated for determination of mlcrogram per llter levels of calcium, magneslum, strontium, and slllcon In saturated sodium chlorlde feed brlnes for electrolytlc chloraikail cells. The effects of matrix modiflcatlon, types of graphite tubes, and furnace temperature programs are described. Procedures to mlnlmlre contamination are also discussed. It is feasible to boil off the NaCl matrix during the char cycle wlthout loss of Ca, Sr, or SI. Best results for Mg were obtalned by deposltlon of the sample Into a pyrolytlc graphite boat (L'vov platform) placed Inside the furnace tube.
Since the first application of Nafion perfluorinated membranes in electrolytic chloralkali cells, it has been recognized, as in the earlier diaphragm-cell and mercury cell processes, that polyvalent cations in the brine feed are deleterious to long-term operation of the cells (1-7). High-purity feed brine is required to avoid precipitation of hydroxides or carbonates of Ca, Mg, Sr, Si, and the like on or within the membrane separators. Precipitation of these species lowers the current efficiency of the cells and eventually leads to physical disruption of the membrane. It is recommended that total hardness of the brine be less than 50 Fg/L (as Ca) to minimize these problems. A variety of processes is used for removing polyvalent ions from brines, but there have not been reliable analyses for very low levels of these ions in saturated brines. It should be feasible to preconcentrate the species of interest by solvent extraction with the aid of ammonium pyrrolidine dithiocarbamate (APDC) or by ion exchange with a resin such
as Chelex-100 (8) and then analyze by flame or graphite furnace atomic absorption spectrophotometry. However, we opted for the relative simplicity of direct determination by graphite furnace atomic absorption spectrophotometry (GFAAS). The sensitivity of GFAAS is more than adequate to achieve microgram per liter detection limits, even with 10to 20-fold dilution of the brine. The extra sample handling in a complex extraction procedure introduces a greater risk of contamination, and it is difficult to ensure that 100% recovery of the analyte is achieved by extraction or ion exchange. Several workers have used GFAAS for determination of Ba in a variety of samples (ref 9 and 10 and references therein), but little has been published on determination of other alkaline earth metals by GFAAS. Berggren et al. (11) measured Ca in mouse pancreas by inserting 2-20 Fg of the dried tissue directly into the graphite furnace tube. Parker et al. (12) determined low levels of Ca in red blood cell suspensions while Bek et al. (13)measured Sr in blood serum. Suzuki and Ohta (14) investigated a molybdenum microtube atomizer for determination of Ca and Sr by atomic absorption and atomic emission. A graphite furnace has also been used to determine low levels of Ca in snow (15). Recently Smith and Cochran (16,17) described a GFAAS procedure for quantitation of Ca and Mg in brines. The interference of NaCl on determination of many elements by GFAAS is well documented in the literature on analysis of seawater (18-20) and in several papers on determination of alkaline earth metals (9-11). The interaction between NaCl and analyte is complex, involving both physical and chemical parameters (21-23). Volatile chlorides of the
0003-2700/82/0354-2154$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
analyte can be lost before atomization. A h , chlorine in the gas phase forms nonabsorbing molecular species with VRporized analyte atoms, A separate concern is nonspecific background absorption and scattered light loss from vaporized NaC1. Physical dispersion of the sample resulting from shattering of salt crystals at high temperatures can cause poor sensitivity and erratic results. A common strategy to minimize atomization interferences involves a combination of matrix modification to convert NaCl to more volatile species and temperature ]programming-this procedure allows removal of the matrix before atomization of the analyte (24). Organic acids such as ascorbic and oxalic acids have been used as a flux to minimize crystal formation and improve thermal contact between sample matrix and furnace (22). The L’vov platform, a small pyrolytic graphite plate inserted into the furnace tube, ia effective in reducing halide matrix interferences on the determination of many elements (25-27). The platform is heated indirectly by radiation from the tube walls and by the purge gm. Therefore, the sample is volatilized into an atmosphere which is hotter than the platform itself and fairly constant in temperature. Nonabsorbing molecular chlorides in the gas phase are broken apart into free atoms, thereby decreasing the interference. which are this paper ~ v will e present GFAAS used routinely in ow laboratory for determination of Ca, Mg, Sr, and Si a t microgram per liter levels in saturated brines. We will show that a L’vov platform minimizes interferences in the determination Of Mg Which Could not be eliminated with either uncoated or pyrolytically coated tubes.
EXPERIMENTAL SECTION All of the analyses were performed on a I’erkin-Elmer Model 5000 atomic absorption spectrophotometer with a Model HGA-500 graphite furnace. A Model AS-40 autosampler was used for good precision and for automated operation of tlhe instrument. The sample aliquot was 20 p L in most experiments with both pyrolytically coated and uncoated graphite tubes. In addition, the Perkin-Elmer L‘vov platform was investigahd for determination of Mg. We found that positioning of the autosampler tip above the L’vov platform was much more critical than with a regular furnace tube. If the tip was too high the same aliquot either splattered out of the platform or wicked onto the wall of the tube. If the tip was so low that it penetrated the final sample drop, it displaced enough volume so that some sample flooded over the side of the platform. This problem in adjusting the autosampler was alleviated by bending the tip SO that il, delivered the drop onto the center of the platform and by using a small sample aliquot (10 &I.
Solutions. Contamination plagues the determination of many elements, especially calcium, magnesium, and silicon, at the microgram per liter level. Rigorous cleaning procedures must be followed to minimize this problem and plastic labware should be used in place of glass. In agreement with IJaxen and Harrison (2% we found that soaking 01 repeated rinfiing of labware with dilute HNOs and deionized water gave the best results. Disposable pipet tips and autosampleT cups were also conditioned with nitric acid; reuse of the disposable labware after a brief rinse with nitric acid was in fact preferable to conditioning fresh ones each time. Samples and standards were stored in Teflon FEP-fluorocarbon resin screw-cap bottles. Most sample handling was done in a circulation-free tissue culture hood (Labconco Model 11000) to minimize airborne contamination. Dilutions and additions of standards were done by pipetting the solutions with Gilson “Pipetman” micropipets directly into the autosampler cups to give a total volume of 1.0 mL. Most analyte solutions were prepared by diluting the brine sample 1:10 with appropriate matrix modifier solution. The method of standard additions was used for quantitation. Standards were made from 1.0 g/L certified atomic absorption standards (Fisher). Deionized water was produced by a MilliQ system from Mil. lipore Corp. “Ultrex”grade nitric acid (J. T. Baker) was very low in all elements of interest, though magnesium levels varied sig-
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Table 1. Furnace Temperature Program for Determination of Ca and Sr in Brinea time, s flow, teomp, step C ramp hold rec read base mL/min 1 110 5 25 2 1400 5 20 3 2700 1 5 -3 0 -10 100 4 110 1 4 0 5 2700 1 5 a The following instrument settings were used: wavelength 422.5 nm (Ca), 460.7 nm (Sr); low slit, 0.7 nm (Ca), 0.2 nm (Sr); 6 s read time in Peak height mode with 0 . h tiwe constant. Brine samples were diluted 1:10 with 5% HNO, and a 20 pL sample aliquot was injected into the furnace. nificantly from bottle to bottle. A”onium nitrate solution was prepared from -Ultrex” grade H N O ~and N H ~ O H(J. T. Baker). Saturated NaCl brine solutions were purified by passing though a column of Amberlite IRC-718 cation exchange resin (Rohm and Haas). Procedure. Our experimental approach was first to optimize the furnace temperature program for standard solutions prepared with matrix modifier (FINO3or NH4N03). Char temperature was gradually increased to find the maximum char temperature at which no analyte was lost* Then starting with the Optimum temperature program for standards, we ran brine samples with and without added and varied the conditions to obtain a net absorption signal close to that of the standard alone. part of this process we again determined the maximum char temperature at which no analyte was lost. Argon flow rate during atomization was adjusted to give the best precision with minimum sacrifice of sensitivity. In most cases a long drying time combined with rapid ramping to the drying temperature gave the most reliable results. Slow ramp times (>60 s) in the drying step sometimes caused formation of large salt crystals which then exploded during the char step. When NH,N03 matrix modifier was used, a slow ramping to the ultimate char temperature allowed volatile salts to boil off gradually with minimal physical disturbance to the sample. Background correction was used for all analyses, though the background signal was negligible in most cmes (except for Mg). The “base line offset correction”function (BOC) on the Model 5000 helped compensate for base line drift during a series of measurements. This feature was especially valuable in methods with the L’vov platform. We noted that the platform, which blocks a small portion of the hollow cathode beam, either expands or scatters more light BS the furnace is heated, thus causing a change in the base line. The BOC function helps correct for this, in effect, by rezeroing the instrument near the end of the char step.
RESULTS AND DISCUSSION Calcium. Optimum conditions for determination of calcium in sodium chloride brines are summarized in Table I. The method is a190 suitable for determination of calcium in caustic solutions and potassium chloride brines. Nitric acid is added to achieve some degree of matrix modification. Usually, it is recommended that a large (40-fold) excess of matrix modifier be added to the brine to Conversion Of NaCl to r ” e Volatile fOrmS (19). However, in this case, there was a trade-off between complete matrix modification and the level of Ca impurity found in the HN03. A slight excess of HN03 was used; 26% brine was diluted 1:lO with 5% HN03. Increasing the HNOBconcentration to 10% gave no of the caabsorption signal, The relative boiling points of NaCl (1413 O C ) and CaC1, (1600 O C ) permit most NaCl to be vaporized at 1400 ocbefore any Ca is lost (Figure 1). Very little brine thus remains in the furnace tube by the time the analyte is vaporized and this residual brine exhibits no suppression effects on the Ca absorbance signal when compared to standard solutions in dilute nitric acid. There is also very little background signal during the
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER
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atomization step. Comparable results were obtained when no matrix modifier was used, which agrees with the findings of Smith and Cochran (16,17). However, we found that partial matrix modification helped increase tube life and eliminate build-up of salt on cooler parts of the furnace. Less than 5% of the calcium remains in the graphite tube after the sample is atomized, and most of this can be removed by a subsequent Uburn-outnstep a t the atomization temperature. An additional amount of calcium is removed by adding deionized water to the graphite tube and repeating the analysis temperature sequence. This suggests that some analyte solution seeps into the graphite tube and then leaches out upon subsequent addition of water. Formation of refractory calcium carbide is another explanation for this memory effect. We hoped that use of pyrolytically coated furnace tubes, which are less porous and less prone to carbide formation, would prevent the memory effect in the calcium determination. However, poor precision with the coated tubes gave ambiguous results. Perhaps newer models of coated tubes which are reported to have a more durable pyrolytic coating would be more successful in this application (29). Coincident calibration curves are obtained for 20-pL aliquots of standards in dilute brine and in nitric acid with a typical slope value of 0.0058 (pg/L)-I. The Ca absorbance measured by our method is linear up to 40 pg/L Ca, for both nitric acid and brine solutions. The minimum concentration of Ca in diluted brine, detectable at the 95% confidence level, is 0.4 pg/L for triplicate instrument readings on each of five solutions (30). The detection limit in saturated brine is then 4 pg/L. The precision at the 95% confidence limits for diluted brine is f0.7 at the 3.4 pg/L Ca level. This precision was determined by one person performing ten independent analyses of a brine sample by the method of standard additions with one instrument reading per solution. The greatest error appears to be in sample preparation, not in the instrumental readings. Relative standard deviation (RSD) of five absorbance readings on one solution is 1.0% , while RSD of readings on five separately prepared brine solutions is 7 %. Strontium. The method for the determination of strontium in brines is much the same as that for the determination of calcium. The furnace temperature program and other conditions are presented in Table I. As in the calcium method, nitric acid matrix modifier is added and residual NaCl is vaporized during the 1400 "C char step without loss of Sr (Figure 2). Contamination, though still troublesome, is not as severe a problem here as it is for Ca analysis. The linear range of the strontium method extends to concentrations of 100 pg/L Sr and the detection limit is 1.5 pg/L in dilute brine using a 20-pL aliquot. The precision is f 6 at the 24 pg/L Sr level for ten independent analyses of a brine sample by the method of standard additions with one reading per solution.
TEMPERATURE
IT)
Figure 2. Effect of char temperature on Sr absorbance signal. The char temperature of the program in Table I was varied for a brine solution diluted 1:lO with 5 % HNO, and spiked with 100 pg/L Sr.
Table 11. Furnace Temperature Program for Determination of Si in Brinea step
teomp, C ramp hold
1
110
2
1400
3 4
2800
5
2800
100
25 25 1 1 1
time, s
rec
read
-3
0
flow, base mL/min
20 10
5 5 5
-10
50
0
a The following instrument settings were used: wavelength 251.4 nm; low slit, 0 . 2 nm; 6 s read time in peak height mode with 0.3 s time constant. Brine samples were diluted 1 : l O with 10% NH,NO, and a 20 p L sample aliquot was injected into the furnace.
Silicon. The furnace temperature program and other instrumental settings for the determination of silicon in brines are shown in Table 11. Again the temperature program is similar to that for the determination of calcium, but NH,N03 is used in place of HNOBfor matrix modification. The pH of the analyte solution has a drastic effect on sensitivity. Optimum pH is approximately 5.0 and use of NH4N03helps to buffer the solutions to this range. Higher or lower acidity causes almost total suppression of the Si absorption signal. Several explanations are plausible. Extremes of pH could cause precipitation of Si species from the analyte solution, so that Si is never introduced into the furnace. The relative concentrations of volatile SiC14and SiC12could be influenced by pH. Also, silicon readily forms refractory silicon carbide which is difficult to atomize. Other workers found that carbide formation depends on the partial pressure of oxygen in the graphite tube, but they did not investigate the effects of pH (31). Carbide formation can be decreased and sensitivity thus improved by using pyrolytically coated graphite tubes. In our hands the coated tubes enhanced sensitivity but peak shape was less reproducible than with uncoated tubes and became more irregular as the coated tube aged. Recent papers on the determination of Si recommend relatively high char temperatures (11550 "C) and long char times ( > 2 min) (31,32). However, we found that in samples containing brine, Si was lost at temperatures above 1500 "C (Figure 3). This is probably due to interference from residual chlorine in the furnace tube, which either forms SiC12or acts as a catalyst for the formation of volatile silicon compounds (31). Our method for determination of Si has a precision of *20 at the 55 bg/L Si level in diluted brine; one technician performed 10 independent analyses of a brine sample by the method of standard additions, with one instrument reading per solution. As in the calcium method, the greatest source
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 -
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Flgure 3. Effect of char temperature on Si albsorbance signal. The char temperature of the program in Table I1 was varied for a brine solution diluted 1:lO with 10% NH4NOBand spiked with 150 pglL Si.
Figure 4. Determination of Mg using a pyrolytically coated tube. The net absorbance signals for Mg in 3% NHINOB (0)and in brine diluted 1:lO with 3% NH,NO, (A) are shown. A 20-pL aliquot was used.
of error is in sample preparation. The absorbance is linear up to 150 pg/L Si and the detection limit in diluted brine is 8 pg/L Si using a 20-pL aliquot. Magnesium. Magnesium proved to be the most difficult of the four elements to determine by GFAAS. Atomic absorption is inherently more sensitive to Mg than to the other three elements, so contamination and variability of the blank signals are more of a problem. Furthermore, the interference of NaCl on Mg is most severe and hardest to eliminate because Mg(C1)2(bp 1412 "C) is just as volatile a0 NaCl (bp 1413 "C). A partially satisfactory analysis was devised by using a pyrolytically coated graphite tube instead of an uncoated tube, by adding NH,N03 for matrix modification, and by including two char steps in the furnace temperature program. The magnesium absorption signal exhibited a cleaner, more reproducible peak shape with a pyrolytically coated graphite tube than with an uncoated tube. Often double peaks occurred with the uncoated tube, and the base line usually dipped below zero absorbance both beforle and after the peak. With the coated tube, the Mg signal was seen as a sharp peak a t the beginning of the atomization step and could be time resolved from a later, broad peak due to nonspecific absorption. Gentle heating through two char steps (400 "C and 1100 "C) eliminated the brine and salts from matrix modification better than a single step to the higher char temperature. Despite the matrix modification and optimization of the temperature program, the NaCl interference persisted as evidenced by depression of brine absorption signals relative to standards prepared in matrix modifier solution (Figure 4). Higher char temperatures helped eliminate more NaCl and produced a lower background signal, but Mg was lost in the process. In both coated and uncoated tubes, and with or without addition of NH,N03 matrix modifier, we noted poor recovery of Mg spikes from brine samples at char temperatures greater than 1100 "C. With lower char temperatures there was a great deal of smoke at the beginning of atomization, especially when no matrix modifier was added. These results are not consistent with the work of Smith and Cochran (16,17). They found good recovery of the spikes of magnesium from saturated brine, using two char steps (190 "C and 1450 "C) and uricoated tubes in an HGA 500 furnace. They also claimed good recovery of other metals with low boiling chlorides, including Fe, Mn, and Cr, and they suggested that either analyte compounds (16) or the alkali metal chlorides ( 17 ) are changed in some way in the furnace. We used their temperature program for Mg as written (16) and with minor adjustments in char time and temperature to compensate for instrument to instrument variations in tempera-
Table 111. Furnace Temperature Program for Determination of Mg in Brine with the L'vov Platform' step 1
2
temp, "C ramp hold 270 1200 2400 110
5 60 0 5
time, s rec read
flow, base mL/min
60
25
6 -3 0 -10 20 4 10 0 a The following instrument settings were used: wavelength 285.2 nm; low slit, 0.7 nm; 6 s read time in peak height mode with 0.3 s time constant. Brine samples were diluted 1:lO with 3% NH,NO, and a 1 0 pL sample aliquot was injected into the furnace. 3
ture calibration, but we never achieved total recovery of added standards from brine samples. Since the char temperature adjustment is apparently critical, and since the Mg signal depends on the surface from which it is atomized, it is possible that lot to lot variation in graphite furnace tubes could account for part of the discrepancy between our results and those of Smith and Cochran. I t is also conceivable that their brine contains species not present in our brine which prevent volatilization of Mg until higher temperatures. We have found that the L'vov platform minimizes interference of residual NaCl matrix on determination of Mg. Good recovery of Mg spikes from brine is achieved and a typical slope value is 0.12 (pg/L)-l for calibration curves in both dilute brine and NH4N03 solutions using a 10-pL aliquot. The sensitivity is less than that shown in Figure 4 due in part to a use of a smaller aliquot with the platform. However, we also noted that sensitivity varies significantly when the platform is repositioned in the tube. When a platform is left in position, precision is h0.6 at the 1.1pg/L Mg level for ten independent analyses of diluted brine by the method of standard additions. The typical detection limit is 0.25 pg/L in diluted brine for triplicate readings on five separate solutions and the absorbance is linear up to 2 pg/L Mg, using a 10-pL aliquot. Table I11 lists the furnace temperature program and other conditions for the method. We found that matrix modification with NH,N03 improved the sensitivity compared to use of H N 0 3 or no matrix modifier. In the dry step, a short ramp time combined with a long dry time gave the best precision and recovery of Mg. A long ramp time caused large crystals to form, which later exploded in the char step. Crystal formation along the edges of the platform was often visible and was especially noticeable when a 20-pL rather than a 1O-wL aliquot was used. The optimum char temperature falls in a
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Anal. Chem. 1982, 5 4 , 2158-2161
very narrow range. Below 1100 "C there is copious smoke during atomization which cannot be accommodated by the background corrector. A significant amount of Mg is lost a t char temperatures above 1200 O C . By using the "Max Power" feature of the HGA 500 furnace which provides very rapid ramping to the atomization temperature, we obtained better precision and better sensitivity than by using a 1.0-s ramp time. The faster heating rate reduces interferences by rapidly producing a constant temperature environment in the tube during the atomization (33).
LITERATURE CITED Dotson, R. L. US. Patent 3793 163, 1974. Seko, M. Japan Patent Applic. Kokai 51-88100, 1978. Seko, M. US. Patent 4 178218, 1979. Suhara, M.; Arai, T. U.K. Patent Applic. 2 047 271. Oda, Y.; Suhara, M.; Goto, S.;Hukushlma, T.; Miura, K.; Hamano, T. German Patent 2 819 527, 1978. Oda, Y.; Suhara, M.; Goto, S.; Hukushima, T.; Miura, K.; Hamano, T. U.S. Patent 4 202 743, 1980. "Hypochlorite Generator for Treatment of Comblned Sewer Overflows"; Water Pollution Control Research Series; U S . Environmental Protection Agency, PB 211 243, 1972. Sturgeon, R. E.; Berman, S. S.;Desaulniers, A.; Russell, D. S. Talanta 1880, 2 7 , 85. Jaslm, F.; Barbooti, M. M. Talanta 1981, 2 8 , 353. Epstein, M. S.;Zander, A. T. Anal. Chem. 1979, 5 1 , 915. Berggren, P.-0.; Berglund, 0.; Hellman, B. Anal. 6iochem. 1978, 8 4 , 393.
(30) (31) (32) (33)
Parker, J. C.; Gitelman, H. J.; Glosson, P. S.;Leonard, D. L. J . Gen. Physiol. 1975, 6 5 , 84. Bek, F.; Janouskova, J.; Moldan, B. At. Absorpt. Newsleft. 1974, 13, 47. Suzuki, M.; Ohta, K. Talanta 1981, 2 8 , 177. Cragin, J. H.; Herron, M. M. At. Absorpt. Newsl. 1973, 12, 37. Smith, M. R.; Cochran, H. B. At. Spectrosc. 1981, 2 , 97. Smith, M. R.; Cochran, H. B. US. Patent 4308030, 1981. Ediger, R. D.; Peterson, G. E.; Kerber, J. D. At. Absorpt. Newsl. 1974, 13, 61. Segar, D. A.; Cantillo, A. Y. Anal. Chem. 1980, 5 2 , 1766. Sturgeon, R. E.; Berman, S. S.; Desaulniers, A.; Russell, D. S.Anal. Chem. 1979, 5 1 , 2364. Churella, D. J.; Copeland, T. R. Anal. Chem. 1978, 5 0 , 309. Hydes, D. J. Anal. Chem. 1980, 5 2 , 959. Guevremont, R. Anal. Chem. 1980, 5 2 , 1574. Ediger, R. D. At. Absorpt. Newslett. 1975, 14, 127. L'vov, B. V. Spectrochlm. Acta, Part 8 1978, 336, 153. Slavln, W.; Manning, D. C. Anal. Chem. 1979, 5 1 , 281. Carnrick, G. R.; Slavin, W.; Manning, D. C. Anal. Chem. 1981, 5 3 , 1866. Laxen, D. P. H.; Harrlson, R. M. Anal. Chem. 1981, 5 3 , 345. Slavin, W.; Manning, D. C.; Carnrick, G. R. Anal. Chem. 1981, 5 3 , 1504. Peters, D. G.; Hayes, J. M.; Hieftje, G. M. "Chemical Separations and Measurements"; Saunders: Philadelphia, PA, 1974; Chapter 2. Frech, W.; Cedergren, A. Anal. Chim. Acta 1980, 113, 227. Muller-Vogt, G.; Wendl, W. Anal. Chem. 1981, 5 3 , 651. Siavin, W.; Manning, D. C. Spectrochim. Acta, Part 6 1980, 356, 701.
RECEIVED for review May 7, 1982. Accepted July 29, 1982.
Determination of Lithium Isotopes at Natural Abundance Levels by Atomic Absorption Spectrometry Allen L. Meler U S . Geological Survey, MS 973, Box 25046, Federal Center, Denver, Colorado 80225
The relationships of the absorption of 'Ll and 7Li hollow cathode lamp emlsslons are used to determine ilthlum Isotopic composition In the natural abundance range of geologic materlais. Absorption was found to have a nonlinear dependence upon total ilthlum concentratlon and Isotopic composltlon. A method using nonlinear equatlons to describe the relatlonshlp of the absorption of 'Li and 7Li lamp radlation Is proposed as a means of caicuiatlng Isotopic composition that Is Independent of total llthium concentration.
The determination of the occurrence and relative concentration of stable lithium isotopes in natural materials is of interest in geochemical exploration as a guide to hydrothermal alteration. It may also have useful application in the exploration for ore deposits and geothermal reservoirs (I). Lithium-6 is apparently enriched in rocks associated with hydrothermal alteration (I). Isakov et al. (2) reported an increase in lithium-6 as the degree of rock alteration increased. The average abundance of lithium in the earth's crust is about 20 ppm, ranging from 5 to 200 ppm in soils and averaging 10 ppm in basalt and 60 ppm in shale (3). Some lithium minerals may contain as much as 4.7% lithium. With the exception of these minerals and highly anomalous samples, the expected lithium concentration range for most geologic samples is about 10-200 ppm. The average natural isotopic abundance of lithium is 7.42% and 92.58% 7Li (4). Some controversy exists over how much natural variation in isotopic composition exists. Svec and Anderson (5) summarized published values, determined
by mass spectrometry, which showed isotopic abundance variation in lithium reagents and lithium separated from various minerals to be from 6.90% to 7.98% 6Li. They attributed this variation to instrumental factors as well as natural variation. In their study of lithium-bearing minerals, variations of only 7.34% to 7.61% 6Li were found. Isakov et al(2) reported a range of 6.77% to 9.28% 6Li in mica samples analyzed by mass spectrometry. Natural water samples analyzed by atomic absorption were reported to have as much as 13.38% 6Li (6). Divis and Clark (I) reported values, determined by atomic absorption, of 7.30% to 27.4% "i in hydrothermally altered rocks and an average of 9.38% 6Li in 24 unaltered volcanic rocks, with a standard deviation of 1.94. The values for the hydrothermally altered rocks have the greatest reported variation in natural lithium isotopic abundance that is readily found in the literature. The possibility of determining isotopes by atomic absorption was suggested by Walsh (7). The basis for the atomic absorption determination of lithium isotopes is that each isotope emits a doublet a t the 670.8-nm resonance line of lithium. The two peaks of each doublet are separated by 0.015 nm, and the doublet of one isotope is shifted by 0.015 nm with respect to the other. The upper wavelength of the 7Li doublet overlaps the lower wavelength of the 6Li doublet as a result of the isotopic shift and doublet separation being equal. The separate wavelength components cannot be resolved by conventional atomic absorption spectrophotometers; therefore, the isotopes cannot be determined independently. However, the difference in absorption of emissions of monoisotopic lamps by the two isotopes provides a means of estimating lithium isotopic composition (I, 6-12). The most commonly
This article not subJect to U.S. Copyright. Published 1982 by the American Chemical Society
